The trans 4-amino-cyclohexyl acetic acid and its derivatives are excellent starting materials for the synthesis of active pharmaceutical agents, therefore it is important to provide an economical process by which the trans 4-amino-cyclohexyl acetic acid and its derivatives can be prepared easy-to-make manner in the required drug-purity and with good yields. For the synthesis of active pharmaceutical agents only the optical (stereomerically) pure trans isomer form may be applicable.
Izvesztiya Akademii Nauk SSSR, Seriya Khimicheskaya (10), 2374-9 (Russian) 1980 discloses a process for the preparation of cis and trans isomers of 4-amino-cyclohexyl acetic acid and their derivatives. Accordingly, cis and trans isomers of 4-amino-cyclohexyl acetic acid are obtained starting from 4-nitrophenylacetic acid sodium salt by a hydrogenation reaction at 130° C. and 150 at pressures in the presence of Raney-Ni catalyst. The products obtained are isolated in the form of hydrogen chloride salts.
According to Wustrow at al (Journal of Medicinal Chemistry, 1998 vol. 41 No. 5 768.) trans 4-amino-cyclohexyl acetic acid ethyl ester hydrogen chloride salt is obtained from 4-nitrophenyl acetic acid by hydrogenation. The hydrogenation of a sodium salt is carried out at first in an aqueous medium in the presence of Raney-Ni catalyst at 49° C. and under 130 at pressures, then further at 130° C. and under 172 at pressures. The 4-amino-cyclohexyl-acetic acid obtained consists of about 81% of trans and 19% of cis isomers. Isolation of the required trans isomer is difficult to manage, as the mixture of the isolated trans and cis 4-amino-cyclohexyl acetic acid is dissolved in ethanol and saturated with anhydrous hydrochloric acid gas and heated to reflux. After cooling the mixture is filtered, the filtrate obtained is concentrated and the trans product is precipitated by ether.
The common disadvantages of the above known procedures are e.g. that the processes can be carried out only at very high temperatures and pressures in the presence of the very pyrophoric Raney-Ni catalyst therefore the industrial procedures are uneconomical and dangerous and require extra equipments and extreme conditions. Further disadvantage is that a sodium salt is hydrogenated therefore the working-up and recovering steps are difficult to manage. Namely, the trans 4-amino-cyclohexyl acetic acid ethyl ester hydrochloric acid salt is produced in anhydrous conditions in ethanol, saturated with hydrochloric acid in addition the reaction mixture is heated to reflux and the product is precipitated with ether. The procedure is disadvantageous from environmental point of view due to use of the very corrosive hydrochloric acid and inflammable ether. Our aim was to provide a safe and easy-to-handle process for the preparation of trans 4-amino-cyclohexyl acetic acid or well isolable derivatives thereof in industrial scale by which the product can be prepared via simple reaction steps and besides that said steps require neither solvents that are highly flammable and/or corrosive nor extra equipments.